There have been two types of the materials which have both EMI shielding properties and light transmitting properties. One is a metal wire mesh, a fiber net with a metalized fiber surface deposited by electroless plating, and a laminate thereof on a transparent substrate such as glass and transparent resin (e.g. acrylic resin, polycarbonate). The other is a transparent substrate having thereon a transparent electroconductive coating such as SnO2, ITO, sputtered gold or other electroconductive oxides.
The former has a limitation in the selectiveness of the diameter or pitch of the mesh wires and fibers in order to transmit visible light, and results in less applicability to conventional use. In order to realize high contrast images and to reduce light reflection on the metal surface of the mesh, a black coating is additionally formed on the metal surface. The latter does not have good shielding effect and is difficult in application to conventional use. If the electroconductivity of the transparent coatings is made high, the light reflection from the coating surface increases, which reduces the visibility.
Therefore, it is desirable that the shielding materials for EMI have both light transparency and high shielding effects. In order to satisfy the requirement, it has been proposed to electrolessly deposit metal mesh on a transparent substrate. The electroless plating on a plastic substrate is one of well known technology in the field of the chemical industries. One of the typical methods of electroless plating onto the plastic substrate is described in U.S. Pat. No. 4,670,306, to Salem. The Salem patent suggests a method for the treatment of a substrate for electroless metal plating which includes the steps of applying onto a substrate a material selected for having adequate adherence to the substrate and for being suitably absorptive of an electroless plating catalyst. The substrate is immersed in a strong acid bath to render the surface hydrophilic, and to provide anchoring effect. However, the Salem patent also points out that the method of etching the substrate leads to too much surface roughness.
In order to avoid such direct etching with strong acid, a porous coating method is described in U.S. Pat. No. 4,244,789 to Coll-Polagos, which discloses that a substrate is coated with a hydrophilic composition and then treated with water to form a porous coating as an undercoating layer for 25 electroless plating. The electroless plating, however, has rarely been applied on a transparent substrate, because it is not necessary that the substrate to be electrolessly plated is transparent. Persons involved in this field always concentrate on the adhesiveness between the surface of the substrate and the electroless plated layer. Accordingly, the substrate for the electroless plating is believed to be translucent or non-transparent.
The production of a material having both light transparency and highly electromagnetic shielding properties is described in Japanese Kokai Publication Hei 5 (1993)-16281 which was filed on Feb. 28, 1991 by the present inventors. The process of the Japanese Publication comprises (i) forming a hydrophilic transparent resin layer on a transparent substrate, (ii) immersing it in a catalyst solution for electroless plating to disperse the catalyst into the hydrophilic transparent resin layer or the surface thereof, (iii) immersing the substrate in an electroless plating bath to conduct electroless plating, thus forming a metal layer on the hydrophilic transparent resin layer and forming metal particles in the hydrophilic transparent resin layer, (iv) forming a resist layer having a desired pattern on the metal layer, (v) removing the metal layer and the metal particles in the non-resist area by etching. Although the method is useful for producing the transparent shielding materials for EMI, the method does not always provide the transparent shielding material with high yield and is not a stable method. And also, the method has not paid any attention to the adhesion between transparent substrate and the hydrophilic transparent resin layer. It is therefore expected to stably reproduce the black region in the hydrophilic transparent resin layer on the transparent substrate by means of electroless plating.